1. Field of the Invention
The invention relates to the field of image sensors and is particularly, but not exclusively, useful in the field of image sensing systems designed to discern changes in an image being sensed.
2. Prior Art
There are many situations in which it is desirable to electronically sense images. In some circumstances it is desired to perceive an image as a whole at a selected moment in time. In other circumstances the primary desire is to discern changes in the image during a selected period of time rather than to perceive the entire image at a given moment.
There are many image sensors in the prior art. However, most, if not all, of them which are capable of sensing a complete image require elaborate post-sensing data processing in order to discern image variations. One prior art technique for discerning image variations has been to sense an initial image frame of the scene to be viewed and store that initial image frame in an external memory and use that initial image as a reference to compare against subsequently sensed image frames of the same scene to determine any differences between the initial frame and the subsequent frames. Subtracting the detected image intensity of the subsequent frame from the detected image intensity of the initial frame on a picture element by picture element basis yields a non-zero result for each picture element which received detectably different intensities of response-inducing radiation in the two frames. Such systems are highly complex in that they require a memory capable of storing an entire electrical representation of an initial image frame without suffering a significant loss of intensity resolution and accuracy. The data processing system must be able either (1) to store the electrical representation of the subsequent image frame and then compare that electrical representation with the electrical representation of the initial frame or (2) to compare the electrical representation of the initial frame with the electrical representation of the subsequent frame in real time as the electrical representation of the subsequent frame is generated or received. Whichever of these techniques is utilized, resolution and accuracy are reduced. If digital memories are used, digitization reduces resolution and accuracy. If analog memories are used, the data from the two frames pass through different devices having non-identical characteristics and the resolution and accuracy are reduced. Such systems, although complex and expensive can be effective in achieving their intended purpose where the scene change which is to be discerned is of an intensity level which makes the change easy to discern. However, due to the complex nature of the signal processing required by these systems, their sensitivity to scene changes which are of a low intensity level or involve a small change in a high intensity level is less than optimum.
An image sensor and image sensing system suitable both for perceiving entire images and for moving target or image change discernment is needed which has a reduced complexity and cost and an improved sensitivity to small intensity changes and to the movement of low intensity targets in a high intensity background.